It is the 12th year in a row that Roger Federer has made the end-of-year finals. Photograph: Georgios Kefalas/EPA

Roger Federer insisted he is happy with his form as he arrived in London for the ATP World Tour Finals.

The 32-year-old has endured his worst season for more than a decade and secured his place in the season-ending tournament only last week as the sixth of eight qualifiers.

It is the 12th year in a row Federer has made the finals but he faces a real challenge to get out of a group also featuring Novak Djokovic, Juan Martín del Potro and Richard Gasquet.

Federer will begin his campaign at the O2 Arena against Djokovic on Tuesday, his 10th match in two weeks across three cities. "It's been a lot of tennis. I was hoping for that problem to occur so I'm happy it's gone this way. I'm happy with my game and the confidence is back. That can carry you really far, so that's good. I like this court. It has a great feel to it."

Back problems have been at the heart of Federer's troubles. "I feel like it's coming together at the right time for me but it has a different feel because it hasn't been as consistent and as good and as solid as it has been in previous years.

"I'm maybe still a little bit unsure about how high is my level of play. You wonder if things are going to be how they used to be just going out there and playing tennis. Warming up for one minute like I used to do when I was 20.

"You do so much work to be able to be in decent shape and then still you're not feeling great. It becomes quite frustrating and a little disappointing.

"But I always see the glass half full, I'm a very positive thinker."

The round-robin stage of the tournament begins on Monday, with the No5 seed Tomas Berdych taking on the finals debutant Stanislas Wawrinka, before the in-form Del Potro plays Gasquet.